Whether in Helsinki or elsewhere, a summit meeting held by any American president with a Kremlin counterpart has proved to be a momentous occasion, particularly during the Cold War when such talks held the promise of staving off Armageddon.

Not that they always went well.

In June 1961, newly elected President John F. Kennedy brushed off the need for a preset agenda during two days of meetings in Vienna with Nikita S. Khrushchev, the Soviet premier. Kennedy’s secretary of state, Dean Rusk, said he had worried that the president was ill-prepared for the steamrollering he received from Mr. Khrushchev on issues ranging from control over a divided Berlin to nuclear arms.